This disclosure generally relates to systems for carrying payload across surfaces, such payload including (but not limited to) sensors used in nondestructive evaluation (NDE) and other types of tools. In particular, this disclosure relates to remotely operated systems for carrying tools, such as NDE sensors, through long tunnel-like passageways and areas with limited access.
Non-destructive inspection of structures involves thoroughly examining a structure without harming the structure or requiring significant disassembly of the structure. Non-destructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. For example, non-destructive inspection is commonly utilized in the aircraft industry to inspect aircraft structures for any type of internal or external damage to the structure. Among the structures that are routinely non-destructively inspected are composite structures. As such, it is frequently desirable to inspect composite structures to identify any flaws, such as cracks, voids, or porosity, which could adversely affect the performance of the composite structure.
Various types of sensors may be utilized to perform non-destructive inspection. One or more sensors may move over the structure to be examined, and receive data regarding the structure from which internal flaws can be identified. The data acquired by the sensors is typically processed by a processing element, and the processed data may be presented to a user via a display.
Accessibility to the structural features requiring inspection is one consideration in choosing a non-destructive inspection device. Access to the structural feature requiring inspection may be so limited that a manual inspection by a technician is not possible. An example of a structure with limited access is an internal joint of a wing structure. More specifically, the bond lines produced by close-out joints, created when the last sections of the wing are attached, exemplify the limited-access features of a structure. Limited-access features of a structure, such as the close-out joints, are difficult to fully inspect.
Another example of a structure with limited access is the internal structure of an airplane composite horizontal stabilizer. Ultrasonic NDE sensors can be used to inspect horizontal stabilizer internal vertical support elements or webs, called “spars”, and the filleted join regions between each spar and top and bottom skins. For this type of inspection, the NDE sensors need to be placed in contact with the surface in the region being inspected. One of the main challenges to performing the inspection is that the areas of interest must be inspected after the horizontal stabilizer has been constructed, which makes most of the areas to be inspected difficult to access.
Magnetic coupling systems for use in the inspection of features within a difficult-to-access space are known. One such system comprises a traction motor-powered “tractor” vehicle that rides on one surface of a skin or panel, which tractor is magnetically coupled to one or more passive “trailer” vehicles riding on another surface of the same skin or panel. The vehicle or vehicles riding on the opposing surface of the skin or panel may be inverted. With this type of magnetic coupling system, the tractor vehicle pulls the trailer vehicle(s) along the desired path.
In the foregoing known magnetic coupling system, the coupling magnets are arranged in multiple North-South pairs on the tractor and trailer vehicles, preferably close enough to each other to provide an attraction force equal to at least the weight of the inverted vehicle(s) plus a safety margin. Magnet pairs produce both normal and tangential (shear) forces between the inner and outer vehicles. These magnets do not touch the skin or panel, but instead are held at a constant distance from the surface which the respective vehicle is in contact with. The amount of separation between each pair of opposing poles of the coupling magnets determines the amount of attachment force in the direction normal to the surface and shear force in the tangential direction. Since the attraction force in magnetically coupled systems is inversely proportional to the square of the separation distance, a relatively small change in the distance between the magnet poles will produce a large change in the attraction force.
A problem arises when the thickness of the skin or panel to which the magnetic coupling system (including at least one inverted vehicle) is mounted varies considerably from one end of the structure to be inspected to the other. The magnetic force has to be enough to keep the inverted vehicle(s) in contact with the skin surface, but it should not be so large that too much friction and rolling resistance is developed for the drive motor to overcome. In addition, too much force on the wheels may damage the skin surface. In order to satisfy these constraints, the magnet separation distance needs to be set within a fairly tight tolerance.
There is a need for system that can actively control the attraction force between the coupling magnets as the vehicles move from one end of a structure to the other end, automatically adapting to the variable thickness of an intervening panel of that structure.